Third-Generation (3G) mobile systems, such as for instance Universal Mobile Telecommunications System (UMTS) standardized within the Third-Generation Partnership Project (3GPP), have been based on Wideband Code Division Multiple Access (WCDMA) radio access technology. Today, the 3G systems are being deployed on a broad scale all around the world. A first step in enhancing this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), both of them providing an improvement of radio access in spectral efficiency and flexibility compared to plain UMTS.
While HSDPA and HSUPA still take the advantage of the WCDMA radio access technology, the next major step or evolution of the UMTS standard has brought a combination of Orthogonal Frequency Division Multiplexing (OFDM) for the downlink and Single Carrier Frequency Division Multiplexing Access (SC-FDMA) for the uplink. The new study item which has become later a work item has been named “Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS terrestrial Radio Access Network (UTRAN)”, abbreviated to E-UTRA and E-UTRAN and often referred to as Long-Term Evolution (LTE) since it is intended to cope with future technology evolutions.
The target of LTE is to achieve significantly higher data rates compared to HSDPA and HSUPA, to improve the coverage for the high data rates, to significantly reduce latency in the user plane in order to improve the performance of higher layer protocols (for example, TCP), as well as to reduce delay associated with control plane procedures such as, for instance, session setup. Focus has been given to the convergence towards use of Internet Protocol (IP) as a basis for all future services, and, consequently, on the enhancements to the packet-switched (PS) domain.
A radio access network is, in general, responsible for handling all radio-access related functionality including scheduling of radio channel resources. The core network may be responsible for routing calls and data connections to external networks. In general, today's mobile communication systems (for instance GSM, UMTS, cdma200, IS-95, and their evolved versions) use time and/or frequency and/or codes and/or antenna radiation pattern to define physical resources. These resources can be allocated for a transmission for either a single user or divided to a plurality of users. For instance, the transmission time can be subdivided into time periods usually called time slots then may be assigned to different users or for a transmission of data of a single user. The frequency band of such a mobile systems may be subdivided into multiple subbands. The data may be spread using a (quasi) orthogonal spreading code, wherein different data spread by different codes may be transmitted using, for instance, the same frequency and/or time. Another possibility is to use different radiation patterns of the transmitting antenna in order to form beams for transmission of different data on the same frequency, at the same time and/or using the same code.
The architecture defined in LTE is called Evolved Packet System (EPS) and comprises apart from E-UTRAN on the radio access side also the Evolved Packed Core (EPC) on the core network side. LTE is designed to meet the carrier needs for high-speed data and media transport as well as providing high capacity voice support to the next decade.
The LTE network is a two-node architecture consisting of access gateways (aGW) and enhanced base stations, so-called eNode Bs (eNB). The access gateways handle core network functions, i.e. routing calls and data connections to external networks, and also implement radio access network functions. Thus, the access gateway may be considered as combining the functions performed by Gateway GPRS Support Node (GGSN) and Serving GPRS Support Node (SGSN) in today's 3G networks and radio access network functions, such as for example header compression, ciphering/integrity protection. The eNodeBs handle functions such as for example Radio Resource Control (RRC), segmentation/concatenation, scheduling and allocation of resources, multiplexing and physical layer functions. E-UTRAN air (radio) Interface is thus an interface between a User Equipment (UE) and an eNodeB. Here, the user equipment may be, for instance, a mobile terminal, a PDA, a portable PC, a PC, or any other apparatus with receiver/transmitter conform to the LTE standard. The described architecture is exemplified in FIG. 31.
Multi carrier transmission introduced on the E-UTRAN air interface increases the overall transmission bandwidth, without suffering from increased signal corruption due to radio-channel frequency selectivity. The proposed E-UTRAN system uses OFDM for the downlink and SC-FDMA for the uplink and employs MIMO with up to four antennas per station. Instead of transmitting a single wideband signal such as in earlier UMTS releases, multiple narrow-band signals referred to as “subcarriers” are frequency multiplexed and jointly transmitted over the radio link. This enables E-UTRA to be much more flexible and efficient with respect to spectrum utilization.
In 3GPP LTE, the following downlink physical channels are defined (3GPP TS 36.211 “Physical Channels and Modulations”, Release 8, v. 8.3.0, May 2008, available at http://www.3gpp.org and incorporated herein by reference):                Physical Downlink Shared Channel (PDSCH)        Physical Downlink Control Channel (PDCCH)        Physical Broadcast Channel (PBCH)        Physical Multicast Channel (PMCH)        Physical Control Format Indicator Channel (PCFICH)        Physical HARQ Indicator Channel (PHICH)        
In addition, the following uplink channels are defined:                Physical Uplink Shared Channel (PUSCH)        Physical Uplink Control Channel (PUCCH)        Physical Random Access Channel (PRACH).        
The PDSCH and the PUSCH are utilised for data and multimedia transport in downlink (DL) and uplink (UL), respectively, and hence designed for high data rates. The PDSCH is designed for the downlink transport, i.e. from eNode B to at least one UE. In general, this physical channel is separated into discrete physical resource blocks and may be shared by a plurality of UEs. The scheduler in eNodeB is responsible for allocation of the corresponding resources, the allocation information is signalised. The PDCCH conveys the UE specific and common control information for downlink and the PUCCH conveys the UE specific control information for uplink transmission.
LTE standard supports two different radio frame structures, which are applicable to Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modi of the standard.
The general baseband signal processing in LTE downlink is shown in FIG. 1 (cf. 3GPP TS 36.211 “Multiplexing and Channel Coding”, Release 8, v. 8.3.0, May 2008, available at http://www.3gpp.org and incorporated herein by reference). First, information bits, which contain the user data or the control data, are block-wise encoded (channel coding by a forward error correction such as turbo coding) resulting in codewords. The blocks of encoded bits (codewords) are then scrambled 110. By applying different scrambling sequences for neighbouring cells in downlink, the interfering signals are randomized, ensuring full utilisation of the processing gain provided by the channel code. The blocks of scrambled bits (codewords), which form symbols of predefined number of bits depending on the modulation scheme employed, are transformed 120 to blocks of complex modulation symbols using the data modulator. The set of modulation schemes supported by LTE downlink includes QPSK, 16-QAM and 64-QAM corresponding to two, four or six bits per modulation symbol
Layer mapping 130 and precoding 140 are related to Multiple-Input/Multiple-Output (HMO) applications supporting more receiving and/or transmitting antennas. The complex-valued modulation symbols for each of the codewords to be transmitted are mapped onto one or several layers, LTE supports up to four transmitting antennas. The antenna mapping can be configured in different ways to provide multi antenna schemes including transmit diversity, beam forming, and spatial multiplexing. The set of resulting symbols to be transmitted on each antenna is further mapped 150 on the resources of the radio channel, i.e., into the set of resource blocks assigned for particular UE by a scheduler for transmission. The selection of the set of resource blocks by the scheduler depends on the channel quality indicator (CQI)—a feedback information signalized in the uplink by the UE and reflecting the measured channel quality in the downlink. After mapping of symbols into the set of physical resource blocks, an OFDM signal is generated 160 and transmitted from the antenna ports. The generation of OFDM signal is performed using inverse discrete Fourier transformation (fast Fourier transformation FFT).
The LTE uplink transmission scheme for both FDD and TDD mode is based on SC-FDMA (Single Carrier Frequency Division Multiple Access) with cyclic prefix. A DFT-spread-OFDM method is used to generate an SC-FDMA signal for E-UTRAN, OFT standing for Discrete Fourier Transformation. For DFT-spread-OFDM, a DFT of size M is first applied to a block of M modulation symbols. The E-UTRAN uplink supports, similarly to the downlink QPSK, 16-QAM and 64-QAM modulation schemes. The OFT transforms the modulation symbols into the frequency domain and the result is mapped onto consecutive subcarriers. Subsequently, an inverse FFT is performed is performed as in OFDM downlink, followed by addition of the cyclic prefix. Thus, the main difference between SC-FDMA and OFDMA signal generation is the DFT processing. In an SC-FDMA signal, each subcarrier contains information of all transmitted modulation symbols, since the input data stream has been spread by the OFT transform over the available subcarriers. In OFDMA signal, each subcarrier only carries information related to specific modulation symbols.
FIG. 2 illustrates the time domain structure for LTE transmission applicable to FDD mode. The radio frame 230 has a length of Tframe=10 ms, corresponding to the length of a radio frame in previous UMTS releases. Each radio frame further consists of ten equally sized subframes 220 of the equal length Tsubframe=1 ms. Each subframe 220 further consists of two equally sized time slots (TS) 210 of length Tslot=0.5 ms. Up to two codewords can be transmitted in one subframe.
FIG. 3 illustrates the time domain structure for LTE transmission applicable to TDD mode. Each radio frame 330 of length Tframe=10 ms consists of two half-frames 340 of length 5 ms each. Each half-frame 340 consists of five subframes 320 with length Tsubframe=1 ms and each subframe 320 further consists of two equally sized time slots 310 of length Tslot=0.5 ms.
Three special fields called DwPTS 350, GP 360, and UpPTS 370 are included in each half-frame 340 in subframe number SF1 and SF6, respectively (assuming numbering of ten subframes within a radio frame from SF0 to SF9). Subframes SF0 and SF5 and special field DwPTS 350 are always reserved for downlink transmission. Seven configurations of supported uplink-downlink subframe allocations within one frame are listed in Table 1, wherein D denotes a subframe dedicated to downlink transmission, U denotes a subframe dedicated to uplink transmission and S denotes a special subframe carrying the special fields DwPTS 350, GP 360, and UpPTS 370.
TABLE 1LTE Rel'8 uplink-downlink configurationsUplink-Downlink-downlinkto-Uplinkconfig-Switch-pointSubframe numberurationperiodicity01234567890 5 msDSUUUDSUUU1 5 msDSUUDDSUUD2 5 msDSUDDDSUDD310 msDSUUUDDDDD410 msDSUUDDDDDD510 msDSUDDDDDDD6 5 msDSUUUDSUUD
The physical resources for the OFDM (DL) and SC-FDMA (UL) transmission are often illustrated in a time-frequency grid wherein each column corresponds to one OFDM or SC-FDMA symbol and each row corresponds to one OFDM or SC-FDMA subcarrier, the numbering of columns thus specifying the position of resources within the time domain, and the numbering of the rows specifying the position of resources within the frequency domain.
The time-frequency grid of NRBULNscRB subcarriers and NsymbUL SC-FDMA symbols for a time slot ISO 410 in uplink is illustrated in FIG. 4. The quantity NRBUL depends on the uplink transmission bandwidth configured in the cell and shall fulfillNRBmin,UL≦NRBUL≦NRBmax,UL,where the values and NRBmin,UL=6 and NRBmax,UL=110 define the smallest and largest uplink bandwidth, respectively. The number NsymbUL of SC-FDMA symbols in a time slot depends on the cyclic prefix length configured by higher layers.
A smallest time-frequency resource corresponding to a single subcarrier of an SC-FDMA symbol is referred to as a resource element 420. A resource element 420 is uniquely defined by the index pair (k,l) in a time slot where k=0, . . . , NRBULNscRB−1 and l=0, . . . , NsymbUL−1 are the indices in the frequency and time domain, respectively.
The uplink subcarriers are further grouped into resource blocks (RB) 430. A physical resource block is defined as NsymbUL consecutive SC-FDMA symbols in the time domain and NscRB consecutive subcarriers in the frequency domain. The resource block parameters are shown in Table 2.
TABLE 2Resource block parametersConfigurationNscRBNsymbULNormal cyclic prefix127Extended cyclic prefix126
The relation between the physical resource block number nPRB in the frequency domain and resource elements (k,l) in a slot is given by
      n    PRB    =            ⌊              k                  N          sc          RB                    ⌋        .  
Each resource block 430 thus consists of twelve consecutive subcarriers and span over the 0.5 ms slot 410 with the specified number of SC-FDMA symbols.
Downlink control signalling is carried by the following three physical channels:                Physical Control Format Indicator Channel (PCFICH) utilized to indicate the number of OFDM symbols used for control channels in a subframe,        Physical Hybrid Automatic Repeat Request Indicator Channel (PHICH) utilized to carry downlink acknowledgements (positive: ACK, negative: MACK) associated with uplink data transmission, and        Physical Downlink Control Channel (PDCCH) which carries downlink scheduling assignments and uplink scheduling grants.        
The Physical Downlink Control Channel carries downlink scheduling assignments. Each scheduling grant is defined based on Control Channel Elements (CCE). Each control channel element corresponds to a set of resource elements. In particular, one CCE contains nine Resource Element Groups (REG), wherein an REG corresponds to four Resource Elements (RE). A control region of a subframe with index k consists of a set of a total number NCCE,k of CCEs, numbered from 0 to NCCE,k−1. The control region is distributed over time and frequency control resources. Multiple CCEs can be combined to effectively reduce the coding rate of control signal. CCEs are combined in a predetermined manner using a tree structure to achieve various coding rates as shown in FIG. 5. A PDCCH is an aggregation of either one, two, four or eight CCEs.
In LTE, the PDCCH is mapped to the first n OFDM symbols of a subframe, wherein n is more than or equal to 1 and is less than or equal to three. Transmitting PDCCH in the beginning of the subframe has the advantage of early decoding of the corresponding L1/L2 control information included therein.
In a subframe, multiple PDCCHs can be transmitted. A PDCCH has multiple formats, called Downlink Control Information (DCI) formats. A DCI transports downlink or uplink scheduling information, or uplink power control commands. Upon detection of a PDCCH with DCI format intended for a particular UE in a subframe, the UE decodes the corresponding PDSCH in the same subframe. Moreover, the UE receives PDSCH broadcast control transmissions—namely Paging, RACH Response, and BCCH—associated with DCI formats signaled by a PDCCH in the common search spaces. In addition, the UE is semi-statically configured via higher layer signalling to receive PDSCH data transmissions signaled via PDCCH UE specific search spaces, based on one of the following transmission modes: single antenna port (port 0), transmit diversity, open-loop spatial multiplexing, closed-loop spatial multiplexing, multi-user MIMO, closed-loop Rank=1 precoding, single-antenna port (port 5).
The UE monitors a set of PDCCH candidates for control information in every non-DRX subframe. Here, the monitoring refers to attempts to decode each of the PDCCHs in the set according to all monitored DCI formats. The UE is not required to decode control information on a PDCCH if the channel code rate is larger than ¾, where channel-code rate is defined as number of downlink control information bits divided by the number of physical channel bits on the PDCCH.
The control channels monitored by a UE may be configured by higher layer signalling. The number of CCEs, which are available for control channel assignment, depends on several factors such as carrier bandwidth, number of transmit antennas, and number of OFDM symbols used for control and the CCE size.
The set of PDCCH candidates to be monitored are defined in terms of search spaces, where a search space at aggregation level 1, 2, 4, or 8 is defined by a set of PDCCH candidates as shown in Table 3.
TABLE 3PDCCH candidates monitored by UESearch spaceNumber ofAggregationPDCCHTypelevelSize [in CCEs]candidatesDCI formatsUE-1660, 1, 1A,specific21261B, 24828162Common41640, 1A, 1C,81623/3A
The UE monitors one common search space at each of the aggregation levels 4 and 8. Common search space corresponds to certain number of CCEs on candidate aggregation levels 4 and 8 (cf. last two rows in Table 3). All UEs in the cell shall monitor the common search space.
The UE also monitors one UE-specific search space at each of the aggregation levels 1, 2, 4, and 8. As shown in Table 3, the UE makes several decoding attempts per aggregation level within the UE-specific search space. Assuming two payload sizes (DCI) per aggregation level, one for downlink scheduling assignment and one for uplink grant, the number of decoding attempts per payload size and per aggregation level are: 6 decoding attempts on aggregation level 1+6 decoding attempts on aggregation level 2+2 decoding attempts on aggregation level 4+2 decoding attempts on aggregation level 8. Thus, per payload size there are 16 blind decoding attempts and overall 32 blind decoding attempts to detect PDCCH in UE specific search space. Similarly, there are 12 attempts to detect PDCCH in the common search space. Thus, there are 44 overall attempts to detect PDCCH.
The common and UE-specific search spaces may overlap.
The Physical Uplink Control Channel (PUCCH) carries uplink control information. The supported PUCCH formats are shown in the Table 4.
TABLE 4PUCCH format for LTE Rel'8Number of bitsPUCCHModulationper subframe,formatschemeMbit1N/AN/A1aBPSK11bQPSK22QPSK202aQPSK + BPSK212bQPSK + QPSK22
PUCCH format 1a and 1b are applicable for transport of ACK/NACK only in PUCCH transmission. Resources used for transmission of PUCCH formats 1, 1a and 1b are identified by a resource index nPUCCH(1) from which an orthogonal sequence index noc(ns) and a cyclic shift α(ns,l) are determined, the orthogonal sequence index and the cyclic shift defining the spreading code used in SC-FDMA. In general, the physical resource used for transmission of ACK/NACK depends on various factors such as uplink bandwidth configuration, bandwidth reserved for PUCCH format 2/2a/2b, number of cyclic shifts used for PUCCH formats 1/1a/1b in a resource block with a mix of formats 1/1a/1b and 2/2a/2b, cell specific cyclic shift value, resource block size in frequency domain expressed as number of subcarriers and resource index nPUCCH(1), for PUCCH formats 1/1a/1b.
in accordance with LTE Rel'8 in the TDD mode, there are two possibilities to transport ACK/NACK feedback information in uplink, supported by higher layer configuration:                Default mode: ACK/NACK bundling using PUCCH format 1a or 1b. Here, bundling refers to transmitting a single ACK/NACK signal for multiple PDSCH transmissions, and        ACK/NACK multiplexing using PUCCH format 1b with channel selection.        
FIG. 6 illustrates the TDD ACK/NACK bundling. The ACK/NACK bundling is performed per codeword across multiple downlink subframes 601, 602, 603, and 604 associated with a single uplink subframe. Logical “and” operation is applied to all individual (dynamically and semi-persistently scheduled) PDSCH transmission ACK/NACKs referring to a codeword in the downlink subframes 601, 602, 603, and 604. Consequently, an ACK is transmitted if all bundled downlink subframes transmit ACK. A NACK is transmitted if at least one downlink subframe transmits a NACK. The bundled one or two ACK/NACK bits are transmitted using PUCCH format 1a and PUCCH format 1b, respectively as shown in Table 4.
For a downlink transmission on PDSCH in subframe n, the UE transmits ACK/NACK in subframe m=n+k, where k is given for each downlink subframe by Table 5 for each uplink-downlink configuration mode as introduced in Table 1.
TABLE 5k value for each DL transmissionSubframe nConfiguration0123456789046———46———176——476——4276—4876—483411———7665541211——87765451211—9876543677———77——5
The UE uses a PUCCH resource with resource index nPUCCH(1) for transmission of ACK/NACK in subframe m. The PUCCH resource is linked to the lowest CCE index of the PDCCH of last detected downlink subframe and the corresponding downlink subframe number.
The ACK/NACK bundling is prone to errors caused by missed downlink scheduling assignments. If the UE is not aware (missing) of downlink scheduling assignments in a subframe, the bundled ACK/NACKs might be transmitted incorrectly. In order to overcome this problem, for each set of bundled downlink subframes at least information about the number of the subframes bundled within the set is exchanged between the eNB and UE.
Providing the number of bundled subframes allows detecting of the missing downlink scheduling assignments and thus reduces the unnecessary retransmissions. The information about the downlink scheduling assignments is indicated by Downlink Assignment Index (DAI). The DAI carries within the PDCCH two bits information which enables the UE to detect possibly missed scheduling assignments. In particular, for TDD, the value of DAI denotes the minimum number of dynamic downlink assignment(s) transmitted to the UE within all bundled subframe(s) n. The DAI may be updated from subframe to subframe. Thus, DAI may be seen as a counter of number of previously assigned downlink subframes within the bundling window. The values of DAI are shown in Table 6.
TABLE 7Value of Downlink Assignment IndexDAIMSB, LSBValue of DAI0, 010, 121, 031, 14
Upon reception of PDCCH, the UE compares the number of received downlink scheduling assignments with the value of DAI in order to detect the previously missed DL assignments.
Within the 3GPP the description of “Further Advancements for E-UTRA (LTE-Advanced)” has been currently under study and can be found in 3GPP TS 36.814 available at http://www.3gpp.org and incorporated herein by reference. This study item covers technology components to be considered for the evolution of E-UTRA, e.g. to fulfill the requirements on IMT-Advanced carrier aggregation, wherein two or more component carriers are aggregated. This should enable LTE-Advanced (also called LTE-A) to support downlink transmission bandwidths larger than 20 MHz. An LTE Rel-8 terminal can receive transmissions on a single component carrier only. An LTE-Advanced terminal with reception capability beyond 20 MHz shall be capable of simultaneously receiving transmissions on multiple component carriers. Here, simultaneously means within the same radio frame. For instance, in the TDD mode of LTE-A, different component carrier may be transmitted/received for different subframes. In the FDD mode, transmission/reception of multiple component carrier shall be possible even within the same subframe.
An efficient and reliable mechanism for transmitting the data signal (e.g. PDCCH) and the control signal (uplink, downlink) are necessary for such a system.